Not applicable.
1. Field of Invention
This invention relates to magnetic-field pick-up coils for measuring time-varying magnetic field based on the Faraday""s law of induction.
2. Description of Prior Art
The need for measuring magnetic field arises in scientific, industrial, and other fields. Magnetic-field sensors commonly employed in these applications use a length of electrically conducting wire formed into a geometrical shape suitable for producing a voltage signal in accordance with the Faraday""s law of induction when magnetic field varies with time. Among suitable geometrical shapes the helix is typical of three-dimensional space curves, and the spiral is typical of two-dimensional planar curves. These sensors are often referred to as magnetic-field pick-up coils, or simply magnetic pick-up coils. But the word, coils, is to be understood to mean coupling elements of a general shape rather than just helixes and spirals.
Other devices share structural similarity with the magnetic pick-up coil, but are functionally different on a fundamental level. For example, the inductor is a storage device for magnetic energy. As an electrical circuit element the inductor impedes the flow of electrical current through it by pumping magnetic energy in and out of storage. The magnetic coil is a device that generates magnetic field. The functionality of either device is inseparably tied to magnetic field that is self-generated by electrical current flowing through it. In contrast, the magnetic pick-up coil is a sensing device, and measures ambient magnetic field that originates from sources other than the coil itself.
A magnetic pick-up coil having elements essential for fulfilling its central functionality is illustrated in a perspective view in FIG. 1A. This basic coil has two helixes shown in green and blue, an inter-helix connection shown in black, and two connecting leads shown in red. The black arrow indicates the direction of magnetic field coupled by the coil. These elements are electrically and mechanically joined in series as indicated in the figure. The smaller green helix nests inside the volume enclosed by the larger blue helix, forming two layers of winding, each with multiple winding turns. The two helixes are wound in the same sense so that voltage generated in either helix adds cooperatively to voltage generated in the other, and resultant voltage appears between the two connecting leads. All elements are made from electrically conducting bare wire that is not electrically insulated, and that is stiff enough to hold its own shape without structural support. Magnetic pick-up coils in practical use are variations of the basic coil modified to improve its performance in some ways.
A measure of performance of a magnetic pick-up coil is its sensitivity, or voltage generated for a unit rate of change of coupled magnetic field, which is often expressed in terms of effective coupling area. Another performance measure of a coil is its sensitivity in relation to its physical size, which may be called specific sensitivity.
The basic magnetic pick-up coil of FIG. 1A has poor specific sensitivity for two fundamental reasons. First, winding must use thick wire to hold its geometrical shape. Second, winding layers must be placed wide apart, and winding turns in each layer must be separated from each other in order to keep winding layers and turns from short-circuiting.
A magnetic pick-up coil of a conventional construction in wide practical use is illustrated in a cross-sectional view in FIG. 1B. The coil is made from thin electrically insulated wire and an electrically insulated former. The cross section of a hollow cylindrical former is shown in dark green, and the cross sections of wire are shown as filled circles in pink. A black circle around each wire cross section represents electrically insulating coating. The diameter of the thin wire is greatly exaggerated in this figure for clarity of presentation. The wire is not stiff enough to hold a geometrical shape on its own. Structural support for layers and turns is provide by winding an innermost layer on, and in contact with, the former, and winding each of outer layers over, and in contact with, a layer just underneath. Turns in a layer are also placed close together, and may be in contact with neighboring turns. Electrical isolation between layers and turns is provided by the wire""s insulating coating in this construction. A magnetic pick-up coil must use material for structural support and electrical insulation that is compatible with an environment in which the coil is used.
Magnetic pick-up coils presently in use suffer a number of shortcomings in scientific applications. They are primarily associated with satisfying simultaneous demands for good specific sensitivity and compatibility with a hostile environment present in a scientific facility.
In a plasma fusion reactor, for example, protective first walls inside an ultra-high-vacuum vessel are expected to become extremely hot under massive heat influx from the plasma. Temperatures of their plasma-facing surfaces are expected to reach as high as 1300xc2x0 C. Magnetic pick-up coils will be placed in extremely limited space behind protective first walls, and must be compatible simultaneously with ultra-high vacuum and extremely high temperatures. These coils will also receive intense nuclear radiation from the plasma. Few designs, if any, exist today for magnetic pick-up coils that can meet these demands. In many other scientific applications, simultaneous needs for ultra-high vacuum and high temperatures are frequently encountered, because the vacuum vessel must be baked at high temperatures, often in excess of 350xc2x0 C., to achieve ultra-high vacuum.
Polyimide and PTFE are commonly used for insulating wire as well as for making a former and other support structures in a magnetic pick-up coil. These versatile insulators are suitable for use in a normal environment. But they are unfit for use in ultra-high vacuum because of excessive out-gassing. They cannot be used at high temperatures because they melt. In a combined ultra-high-vacuum and high-temperature environment, vacuum and temperature ranges accessible to them are more severely limited, because out-gassing increases rapidly at elevated temperatures. Polyimide- and PTFE-based magnetic pick-up coils are usually not used at temperatures much above 200xc2x0 C. in ultra-high-vacuum applications. These coils are also susceptible to damage by nuclear radiation. Some hostile environments encountered in industrial applications also make these coils unfit for use. For example, some chemical reactions, heat, abrasion, and nuclear and optical radiation may degrade or destroy their support structure and electrical insulation.
Effort to surmount some of the problems encountered in a hostile environment led to the use of different material and constructions for structural support and electrical insulation. But achieving good specific sensitivity at the same time remains an elusive goal.
Mineral-Insulated cable, or MI cable, is sometimes used for building a magnetic pick-up coil. But the thick cable adds appreciably to the overall coil size, and reduces specific sensitivity. MI cable cannot be bent in small radius, and limits design options. MI cable itself is expensive to manufacture. Coils made of MI cable are also expensive to build, because handling of MI cable is difficult. Wire is also used that is coated with ceramic for insulation, but has shortcomings similar to those of MI cable.
Structural support and electrical insulation are sometimes provided for thin bare wire by inserting a sleeve of an insulating material, typically ceramic, between adjacent winding layers, and placing winding turns in each layer in a helical groove cut into the sleeve. But winding turns in such constructions are wide apart, and sleeves add greatly to the overall coil size. Specific sensitivity is poor. It is technically difficult and economically costly to make a large number of nested ceramic sleeves mechanically stable. Coils made of ceramic sleeves are fragile, and expensive to manufacture.
Metallic film of a spiral or other suitable planar shape may be laid on the surface of a hard ceramic plate using etching and other techniques to make a single-layer magnetic pick-up coil. A multitude of these planar coils in two dimensions may be assembled in a stack, one on top of another, in an effort to build up in a third dimension perpendicular to the plate face, and make a single multi-layer coil with high specific sensitivity. But no convenient and secure ways exist for electrically and mechanically connecting the single-layer coils.
Shortcomings of magnetic pick-up coils of a conventional construction can arise also from other demands made on the coil: for example, the need for simultaneous measurement in multiple directions and electrostatic shielding.
Needs arise often in scientific applications to measure magnetic field in more than one direction about a single point in space. They can be met with a multi-axis coil constructed from nested multiple single-axis coils oriented in multiple directions. But when each of these single-axis coils must be built to be compatible with a hostile environment, with appropriate material and construction for structural support and electrical insulation, the resultant multi-axis coil will be large, and specific sensitivity will be poor.
Electrostatic shielding is often required in a magnetic pick-up coil used in scientific applications, because measurement is conducted in an electrostatically noisy environment. An electrostatic shield is an electrically conducting structure enclosing a magnetic-field coupling element, and shields out undesired noise coming from electrostatic sources by creating a surface of equal electrostatic potential around the coupling element. But an electrostatic shield allows, at the same time, desired time-varying magnetic field to reach the coupling element by eliminating or reducing eddy currents in the shield driven by the field. An electrostatic shield of a conventional construction needs its own structural support and electrical insulation, adds appreciably to the physical size of a magnetic pick-up coil, and reduces its specific sensitivity.
Further shortcomings of a magnetic pick-up coil of a conventional construction are associated with economical manufacturing, testing, and marketing.
Magnetic pick-up coils of a conventional construction are usually not amenable to mass production, and cannot take advantage of the economy of scale in manufacturing. They are fabricated one at a time, and resultant variations in their characteristics necessitate testing and calibration of individual pieces, further adding to production cost. Various limitations described in the above paragraphs make it difficult to let coils of a single design serve in a variety of environments. Coils of many different designs must be prepared for different applications, and increase their marketing cost.
In accordance with the present invention a magnetic pick-up coil comprises magnetic-field coupling elements embedded in a body of ceramic material for structural support and electrical insulation. An electrostatic shielding element may optionally be embedded in the same ceramic body.
Accordingly, several objects and advantages of the present invention are:
1 to provide a magnetic pick-up coil that is suitable for use in a wide variety of environments;
2 to provide a magnetic pick-up coil that is suitable for use in hostile environments;
3 to provide a magnetic pick-up coil that has high specific sensitivity;
4 to provide a magnetic pick-up coil that is compact and rugged;
5 to provide a magnetic pick-up coil that can effectively use space curves in three dimensions as geometry of its coupling elements;
6 to provide a magnetic pick-up coil that can measure in more than one direction about a single point in space; and
7 to provide a magnetic pick-up coil that is amenable to mass production.